Vertical light emitting diodes

ABSTRACT

A light emitting device (LED) employs one or more conductive multilayer reflector (CMR) structures. Each CMR is located between the light emitting region and a metal electrical contact region, thereby acting as low-loss, high-reflectivity region that masks the lossy metal contact regions away from the trapped waveguide modes. Improved optical light extraction via an upper surface is thereby achieved and a vertical conduction path is provided for current spreading in the device. In an example vertical, flip-chip type device, a CMR is employed between the metal bottom contact and the p-GaN flip chip layer. A complete light emitting module comprises the LED and encapsulant layers with a phosphor. Also provided is a method of manufacture of the LED and the module.

FIELD OF THE INVENTION

The present invention relates to vertical, flip-chip light emittingdevices and particularly to the use of conductive multilayer reflectorstructures in such devices.

BACKGROUND TO THE INVENTION

Vertical light emitting diodes typically employ metal substrates and topcontact regions. These characteristically introduce optical loss totrapped waveguide modes residing in the light emitting region.Currently, top surface roughening techniques are employed to extract themaximum amount of light on the first pass of light incident on the topsurface. However, if light is multiply scattered by the bottom metalreflector then loss is introduced to the trapped waveguide mode.

Light emitting diodes (LEDs) are based on a forward biased p-n junction.Recently, LEDs have reached high brightness levels that have allowedthem to enter into new solid state lighting applications as well asreplacements for high brightness light sources such as light engines forprojectors and automotive car headlights. These markets have also beenenabled by the economical gains achieved through the high efficienciesof LEDs, as well as reliability, long lifetime and environmentalbenefits. These gains have been partly achieved by use of LEDs that arecapable of being driven at high currents and hence produce high luminousoutputs while still maintaining high wall plug efficiencies.

Solid state lighting applications require that LEDs exceed efficienciescurrently achievable by alternative fluorescent lighting technologies.The efficiencies of LEDs can be quantified by three main factors, namelyinternal quantum efficiency, injection efficiency, and extractionefficiency. The latter being the basis for the present invention.

One of the main limiting factors reducing the extraction efficiency inLEDs is the emitted photons being totally internally reflected andtrapped in the high refractive index of the epi-material. These trappedwaveguide modes propagate in the LED structure until they are scattered,escape or reabsorbed. The thickness of the LED structure determines thenumber of modes that can be set up.

Many methods have been successfully employed to improve light extractionin LED heterostructures. These include shaping LED die, as described inU.S. Pat. No. 6,015,719 and U.S. Pat. No. 6,323,063, flip-chip mountingof LEDs as described by Wierer et al. in Appl. Phys. Lett., 78, Pg.3379, 2001, roughening of the top surface as taught by Schnitzer et alin Applied Physics Letters 63, 2174, 1993, and using omnidirectionalreflectors as suggested by Fink et al. in Science vol. 282, Pg. 1679,1998. Other suggested methods include the use of periodic texturing onat least one interface of the structure to improve light extraction outof the light emitting region, as suggested in U.S. Pat. No. 5,779,924.

To provide light emitting devices with high current and thermal drivingcapabilities the vertical type n-p contact configuration in GaN materialsystems has been recently adopted. Such examples have been disclosed inU.S. Pat. No. 6,884,646 and U.S. Pat. 20060154389A1. However, one majordrawback with such vertical type light emitting structures is theexistence of optically lossy metal contacts in the close vicinity of thelight emitting heterostructure. Trapped modes in the high index lightemitting device typically undergo multiple internal reflections. Thephotons reflected at the interface between the metallic contact surfaceand the heterostructure material experiences large losses and hencereduces the total light output of the light emitting diode.

In U.S. Pat. No. 6,784,462 the use of an omni-directional reflector isproposed. This single dielectric electrically insulating layer isdisposed between the light emitting region and the lower conductiveregion and having a plurality of electrical conductive vias contactingthe lower light emitting region and an electrical contact. It istypically an object of vertical light emitting devices to provide goodelectrical and thermal conduction. However, the single dielectric layerresiding between the light emitting region and the lower conductiveregion hinders good electrical conduction. In addition, a singledielectric layer will not provide true omni-directional reflectivity andlight at angles residing within the escape cone formed between the lightemitting medium and the dielectric layer will experience a reflection atthe metal contact boundary, which will introduce optical loss.Additional loss in the light emitting device will also be experienced bythe metal electrical contacts at the top surface of the device, which isnot desirable.

Back Light Units (BLU) for LCD panels are key elements in theperformance of an LCD panel. Currently most LCD panels employ compactcathode fluorescent light (ccfl) sources. However, these suffer fromseveral problems such as poor colour gamut, environmental recycling andmanufacture issues, thickness and profile, high voltage requirements,poor thermal management, weight and high power consumption. In order toalleviate these problems LCD manufacturers are implementing LED BLUunits. These offer benefits in improved light coupling, colour gamut,lower power consumption, thin profiles, low voltage requirements, goodthermal management and low weight.

Another application area for the present invention is in light enginesfor front and rear projectors. Conventional High Intensity Discharge(HID) type projector light engines have always been hindered by lowefficiency and short lifetime resulting in slow adoption into consumermarkets.

The present invention is directed towards another technique forimproving the efficiency of LEDs, thereby enabling their use in variousapplications, including those described above.

SUMMARY OF THE INVENTION

The object of the invention is a vertical, flip-chip, light emittingdevice with a conducting substrate or carrier incorporating a lowoptical loss bottom reflector and low optical loss top contact.

According to a first aspect of the present invention, a light emittingdevice comprises:

a first semiconductor layer having doping of a first type;

a second semiconductor layer having doping of a second type;

a light emitting region interdisposed between the first and secondsemiconductor layers;

a first electrode layer disposed proximal to the first semiconductorlayer and distal to the second semiconductor layer;

a second electrode layer disposed proximal to the second semiconductorlayer and distal to the first semiconductor layer; and

a first multilayer reflector stack interdisposed between the firstelectrode layer and the first semiconductor layer, the first multilayerreflector stack comprising at least a first layer disposed proximal thefirst electrode layer and a last layer disposed distal the firstelectrode layer,

wherein:

light generated in the light emitting region is extracted from thedevice through a surface of the second semiconductor layer;

the first multilayer reflector stack extends at least partially across asurface of the first semiconductor layer;

at least 60% of light incident on the first multilayer reflector stackthat is generated in the light emitting region is reflected by the firststack; and

at least the first and the last layer of the first multilayer reflectorstack are electrically conducting and optically transparent.

In the present invention, conductive multilayer stacks (CMS) areemployed to act as low-loss, high-reflectivity regions masking the lossymetal contact regions away from the trapped waveguide modes. These areemployed to provide both improved optical light extraction and avertical conduction path for current spreading in the semiconductorlight emitting device.

Preferably, the novel low loss bottom conductive multilayer reflector(CMR) stack is employed between the metal bottom contact and the p-GaNflip chip layer. Although reference is made to InGaN light emittingdiodes, this is by way of example, and the present invention can beimplemented in other light emitting material systems such as, but notrestricted to, InGaAs, InGaP, ZnO.

Preferably, the top metal contact is deposited on top of another CMRstack to provide for reduced light attenuation from light emitted underthe contact region. The finite top contact region contributes to aminimum of around 1% to 5% of the active top surface light emittingregion and hence if the light emerging from under these regions can beallowed to escape with minimum loss then an increase in the totalluminous output of the LED can be achieved.

Preferably, the bottom CMR stack and metal contact are roughened toallow for increased scattering and reduced specular reflectivity. Thisprovides an increased probability of trapped light to diffusely reflectand reside in the escape cone of the light emitting material (GaN inthis case).

The use of bottom and top CMR stack layers in the present inventionallows the light emitting device to be thinned down to have a totalthickness less than 3 μm. This permits the LED to achieve highextraction efficiency while still maintaining high current injection.

According to a second aspect of the present invention, a light emittingmodule for solid state lighting applications comprises:

a light emitting device according to the first aspect;

a first encapsulating material covering at least the light extractingsurface of the second semiconductor layer;

a second encapsulating material overcoating at least the firstencapsulating material; and,

a phosphor material interdisposed between the first and the secondencapsulating materials.

In this way, a light emitting module is formed by embedding a phosphorin an encapsulating material that resides on top of the light emittingdevice of the present invention. The encapsulation comprises of twolayers of optically transparent environmentally resistant material. Thefirst encapsulation material that is disposed proximal to the lightemitting device is formed of a high refractive index material and istextured or shaped to extract the maximum number of photons. Thephosphor is subsequently disposed on the high refractive indexencapsulant while another encapsulation material distal to the lightemitting device is disposed on the top of the phosphor. The distalencapsulant is also shaped to provide the desired far field emission outof the light emitting module.

According to a third aspect of the present invention, a method ofmanufacturing the light emitting device of the first aspect or the lightemitting module of the second aspect comprises the steps of:

growing each of the second semiconductor material, the light emittingregion, and the first semiconductor material;

depositing a first multilayer reflector stack;

forming mesa isolation trenches in the light emitting device;

depositing a passivation layer;

depositing a first electrode;

attaching a conductive sub-mount;

removing growth substrate;

depositing a second multilayer reflector stack;

depositing a second electrode;

roughening a top surface of the second semiconductor layer; and

separating an isolated light emitting die.

In this way, a light emitting device or light emitting module can befabricated having the desired high performance properties. Further stepsmay be performed to apply a suitable encapsulant and phosphor to thebasic device structure.

BRIEF DESCRIPTION OF THE DRAWINGS

Examples of the present invention will now be described in detail withreference to the accompanying drawings, in which:

FIG. 1 shows a cross section of a first example of the light emittingdevice of the present invention.

FIG. 2 shows a cross section of a second example of the light emittingdevice of the present invention.

FIGS. 3 a and 3 b show a cross section of two embodiments of a firsttype of light emitting diode of the present invention with top surfaceroughening.

FIGS. 4 a and 4 b show a cross section of two embodiments of a secondtype of light emitting diode of the present invention with top surfaceroughening.

FIG. 5 a illustrates the loss experienced by trapped photons incident ona top surface metal contact in a known device.

FIG. 5 b illustrates the minimal loss experienced by trapped photonsincident on a metal contact in a device of the present invention.

FIG. 6 shows a cross-section of a light emitting module of the presentinvention for use in solid state lighting applications.

FIG. 7 a-g illustrate the processing steps for fabricating a lightemitting diode according to the present invention.

DETAILED DESCRIPTION

The present invention provides high wall plug efficiency light emittingdevices by improving both the light extraction efficiency as well as theinjection efficiency. The invention can be incorporated in a lightemitting device of any semiconductor material system such as, but notrestricted to, InGaN, InGaP, InGaAs, InP, or ZnO. A GaN based LED havingan epitaxial layer formed on a sapphire substrate is used as an examplein the present invention.

GaN light emitting devices comprise a p-n junction heterostructure ofrefractive index about 2.45. When the light emitting device is forwardbiased, spontaneously-emitted photons are generated. If the wavevectorof the photon resides below the light line (in a frequency-wavevectordiagram) of the material, the photon is totally internally reflected andtrapped in the high refractive index of the epi-material.

Table 1 below lists the approximate extraction enhancement achievedusing the different techniques commonly employed to extract light fromthe top surface of an unpackaged vertical LED. The enhancement factorgiven represents the multiplicative improvement over the performance ofa reference, bare, unroughened light emitting device. The numbers arebased on an example structure whereby the mirror is assumed to be 100%reflective and the location of the multiple quantum well (MQW) isoptimised to direct most of the light within the light line of thestructure and to achieve a cavity type effect. Only about 4.35% of thelight is extracted out of the top surface of a bare unroughened LED.

TABLE 1 Extraction technique Extraction enhancement Mirror 2.0Microcavity 1.75 Roughening techniques 2.5

The present invention provides an electrically-efficient, verticalflip-chip, thin-film light emitting device with a conducting substrateor carrier incorporating a low optical loss bottom reflector and lowoptical loss top contact. The incorporation of low loss conductivereflectors on the bottom and top contacts allows trapped modes topropagate in the high index light emitting device until they areefficiently extracted by the top surface texturing or by the edgefacets. The reflectors comprise a conductive multilayer stack that isdesigned to efficiently reflect incident light while still maintaininghigh current injection efficiency.

In a first embodiment of the present invention, shown in FIG. 1, a novellow loss bottom conductive multilayer reflector (CMR) stack, 110, isemployed between the metal bottom contact, 101, and the p-GaN flip chiplayer, 102. The layer 101 may also comprise a multilayer of electricaland thermal conducting metals optimised for maximal adhesion and highestoptical reflectivity. The layer 101 resides on top of a conductivesubstrate 100, which also acts as the bottom p-contact. The CMRcomprises at least 3 material layers, but may comprise 4 layers, 5layers or more. The bottom CMR stack comprises transparent conductivematerials. The materials of the layers in the CMR stack are selected andstacked to maximise the dielectric contrast between neighbouring layers.Hence, materials with alternating low and high refractive indexes areemployed in the same stack.

The reflectivity characteristics of the stack can be analysed by usingthe transfer matrix method (TMM) or other similar modelling technique.The CMR can comprise of any combination of multilayer stack design. Itis an object of the invention to implement a high reflectivitymultilayer stack that can reflect the largest percentage of trappedmodes without introducing optical loss. Some examples of possiblemultilayer stack configurations are Omni-directional reflectors (ODR),periodic, aperiodic, binary, or quasiperiodic.

Determining the Power Reflectivity of the CMR Stack

The transfer matrix method for calculating the reflectivity of a CMRstack is described as follows. Firstly, the transmission and reflectionof a single layer in the stack is determined as follows:

$\begin{matrix}{\begin{pmatrix}E_{i - 1} \\H_{i - 1}\end{pmatrix} = {M_{i}\begin{pmatrix}E_{i} \\H_{i}\end{pmatrix}}} & (1)\end{matrix}$

where E and H, respectively, represent the Electric and the Magneticfield residing in the stack, and M_(i) represents the transformationmatrix for the i-th layer in the stack. The transformation matrix isgiven by

$\begin{matrix}{M_{i} = \begin{pmatrix}{\cos \left( {k_{i}d_{i}} \right)} & {{- j}\; Z_{i}{\sin \left( {k_{i}d_{i}} \right)}} \\\frac{{- j}\; {\sin \left( {k_{i}d_{i}} \right)}}{Z_{i}} & {\cos \left( {k_{i}d_{i}} \right)}\end{pmatrix}} & (2)\end{matrix}$

where the wave-vector is defined as k_(i)=2πn_(i)/λ for refractive indexn_(i), and the electromagnetic impedance Z_(i) is defined asZ_(i)=Z_(o)/n_(i), with reference to the impedance of free space Z_(o).The angle θ_(i) defines the propagation angle of the wavevector in thei-th layer. To determine the reflection and transmission coefficients ofthe complete CMR stack with N layers, the product of the transformationmatrix of all the layers is calculated, as follows:

$\begin{matrix}{M = {{\prod\limits_{n = 1}^{N}\; M_{n}} = {{M_{1} \times \ldots \times M_{N}} = \begin{pmatrix}m_{11} & m_{12} \\m_{21} & m_{22}\end{pmatrix}}}} & (3)\end{matrix}$

Equations (1) and (3) are then combined to relate the fields at the fistand Nth layers, as follows:

$\begin{matrix}{\begin{pmatrix}E_{1} \\H_{1}\end{pmatrix} = {{M\begin{pmatrix}E_{N} \\H_{N}\end{pmatrix}} = {\begin{pmatrix}m_{11} & m_{12} \\m_{21} & m_{22}\end{pmatrix}\begin{pmatrix}E_{N} \\H_{N}\end{pmatrix}}}} & (4)\end{matrix}$

Finally, the transmission and reflection coefficients of the completeCMR stack may be calculated, whereby the power reflectivity of the CMRstack is given by:

$\begin{matrix}{R = {\frac{{Z_{N + 1}m_{11}} + m_{12} - {Z_{1}Z_{N + 1}m_{21}} - {Z_{1}m_{22}}}{{Z_{N + 1}m_{11}} + m_{12} + {Z_{1}Z_{N + 1}m_{21}} + {Z_{1}m_{22}}}}^{2}} & (5)\end{matrix}$

The transparent conductive materials in the bottom CMR stack cancomprise multilayers of optically transparent metal oxides or nitridessuch as, but not limited to, ZnO, Indium Tin Oxide (ITO), Titaniumoxide, GaN or Carbon Nanotubes (CNT) or transparent conductive metaloxides with a spinel crystal structure. Such materials typically possesshigh refractive indexes of approximately between 1.8 and 2.45. Thesematerials provide suitable candidates for the high refractive indexlayers in the CMR stack. The index value of the high refractive indexmaterial can also exceed that of the light emitting region, for exampleup to 2.6, or up to 2.8. For the low refractive index layers, metaloxides or nitrides or oxyntrides with higher porosity, such as ITOnanorods, GaN nanocolumns, AlN nanocolumns, ZnO doped with Silicon orMgF can be deposited, providing a low refractive index of around 1.3.Transparent conductive polymer materials can also be used. Examples ofthese include poly(ethylenedioxythiophene):poly(styrenesulfonate)(PEDOT:PSS) or polyaniline:poly(styrenesulfonate) (PANI:PSS), havingrefractive indexes of about 1.5. The refractive index of the lowrefractive index material can reside between 1.0 and that of the indexof the light emitting material, which in the case of GaN is 2.45.

As shown in FIG. 1, the vertical type light emitting device structure ofthe present invention further comprises an n-type GaN layer, 104, aswell as the light emitting Multiple Quantum Well region, 103, sandwichedbetween layer 102 and 104.

In a preferred embodiment of the invention, the top multilayer metaln-contact region, 107, is deposited on top of another CMR stack region,109. The multilayer of metal can comprise a thin adhesion layer toreduce contact resistance followed by a reflective metallic layerfollowed by a metallic electrode.

The top CMR stack is incorporated to reduce the photon absorption andtotal light output attenuation from the light emitted under the topcontact region. The top electrode layer extends across a proportion ofthe area of the light extracting surface of the second semiconductorlayer that is more than 1%, preferably more than 3%, more preferablymore than 5%, even more preferably more than 10%, still more preferablymore than 25%, and most preferably more than 50%. Hence, if the lightemerging from under these regions can be allowed to escape with minimumloss, then an increase in the total luminous output of the LED can beachieved.

The top CMR stack, 109, can comprise of similar transparent conductivematerials used in the bottom CMR stack 110. However, another object ofthe present invention is to provide a CMR stack of mainly dielectricmaterials, 108, surrounded by a transparent conductive material, 106.The layer 108 comprises of at least 3 material layers, but may compriseof 4 layers, 5 layers or more. The multilayer 108 is optimised formaximum omnidirectional reflectivity, while the layer 106 is selectedfor optical transparency and maximal electrical conductivity. Layer 106can comprise of similar materials selected for use in CMR 110, while forlayer 108 a much wider selection of optically transparent dielectricscommonly employed for optical coatings can be selected. These caninclude, but is not limited to, Silicon Nitride or Silicon Dioxide,metal oxides, nitrides or oxynitrides, such as derived from thefollowing metals: Al, Hf, Ta, Ti, Cr, Zr. The conductive layer 106resides on top of the n-GaN surface to provide a good electricalcontact, as well as overcoating the multilayer 106 to provide a path forelectrical current to propagate from contact 107 to the n-GaN material104.

The final light emitting device is surrounded by a passivation layer,105, to protect the GaN material from the environment and fromoxidation. The passivation material may comprise Silicon Dioxide,Silicon Nitride or a polymer layer, although other materials arepossible.

As shown in FIG. 2, in one embodiment the top CMR stack, 206, cancomprise of transparent conductive materials similar to those in bottomstack 110. The CMR stack 206 is designed, and constituent materials areselected in a similar way, to the bottom CMR stack 110. In the exampledescribed in FIG. 2, a 3-layer multilayer stack is implemented. Thefirst layer 200 can comprise, for example, of a low refractive indexmaterial such as doped ZnO, followed by a layer 201 of higher refractiveindex ITO, followed by a layer 202 of a lower refractive index material,e.g. doped ZnO. The thickness of each layer is optimised to providemaximal reflection of incident photons back into the GaN material. Inthis case the transparent conductive surrounding overcoat is notrequired, since in this example the top n-type contact 204 may bedeposited on top of a thin metal layer 203, which will perform similarfunctions to layer 101 in terms of improved adhesion and electricalconductivity.

As an outcome of the low loss reflective top electrodes, it is an objectof the present invention to increase the percentage coverage of the topelectrodes to allow for improved current injection when high currentdensity LED operation is required. The top electrode density isincreased to 5%, preferably 10%, more preferably 15%, and even morepreferably 20% of the total top active surface of the LED device. Theincreased density allows improved current spreading along the n-GaNlayer maintaining high injection efficiency even at high drive currentof, for example, 1 A, 1.5 A, 2 A, 2.5 A, or 3 A. Due to the novel lowloss top electrodes the increased coverage will not to detrimental tothe light extraction efficiency of the LED. An example plan view of thetop electrode is sketched in the insert in FIG. 2. Two square pads atthe bottom edge of the LED device exist for wire bonding. Crossconnected electrodes across the top surface allows for improved evencurrent spreading across the surface of the n-doped material 104. Thespacing as well as width of electrodes directly relates to theefficiency of injecting the carriers at high current into the LEDdevice.

In a preferred embodiment of the invention the top n-doped GaN material104 is roughened to allow for increased probability of light escapingout of the top surface of the LED. FIG. 3 a shows an example of suchroughening, using the device shown in FIG. 1, whereby the upper surface300 of layer 104 is roughened. The roughening process introduces manyscattering centres at the surface of the LED to allow light an increasedchance of escape out of the top surface. The scattering centres increasethe probability of photons incident on the roughened surface 300 lyinginside the GaN-air escape cone angle.

The roughening can take many different forms, including wet-etchedpyramidal or inverted pyramidal structures on the surface of the GaNmaterial, as-deposited clusters of high refractive index opticallytransparent dielectrics, or additionally as perturbed re-growth of ahigh refractive index materials. If small scale roughness, of the orderof the emitted wavelength (˜λ/n_(GaN)) is introduced, trapped modesincident on the surface experience diffuse reflectivity as opposed tospecular reflectivity, thereby increasing their chance of residingwithin the escape cone at the next multiple internal reflection. In apreferred embodiment of the present invention, the top emission surfaceis randomly arranged with protruding pyramids ranging in size betweenapproximately 0.5 μm and 2.5 μm, thereby providing high extractionefficiency for trapped waveguide modes. Top surface extractionenhancement factors of up to 2.5, 3.0, 3.5, 4.0, 5.0, or 6.0 may berealised, as compared to a bare, unroughened, unencapsulated LED withapproximately 4.35% of the light emitted from the top surface.

In FIG. 3 b, another embodiment of a roughened top n-doped GaN surfaceis demonstrated. In this case, the n-GaN top surface region 301 underthe top CMR contact, is also roughened. This allows light that wasspecularly reflected by the top contact region to diffusely reflect backdown, thereby increasing the probability of the light residing in theescape cone at the next incidence on the top roughened surface.

FIGS. 4 a and 4 b show other exemplary illustrations of surfaceroughening employed in the embodiment of the present invention shown inFIG. 1. In the example shown in FIG. 4 a, surface roughness 400 isapplied to a LED with a top and bottom transparent CMR stack. In anotherexample, shown in FIG. 4 b, the surface roughness is also applied to thetop surface of the n-GaN region under the top CMR stack, as indicated by401.

In FIGS. 5 a and 5 b, the benefits of the present invention are comparedwith prior art designs for top metal contacts. In FIG. 5 a, a prior artdevice with a top metal contact is illustrated. The metal contact, 502,adheres to the top of the n-GaN material of the LED structure, 500, byuse of an adhesion layer 501. Electrical injection, 503, and carrierdiffusion into the n-type material is achieved by the direct metalcontact. However, trapped photons 504 incident on the metal contact 502experience a large amount of optical loss, 507, and specular reflection505 is greatly attenuated. By contrast, in FIG. 5 b, an example of theoperation of the present invention is demonstrated. The top transparentCMR stack 514 resides between the metal contact 512 and the LED n-typeGaN material 500. A metal adhesion layer 511 between the 514 and 512allows for improved adhesion between the metal contact and the 514 CMRstack. In this case trapped photons 515 incident on the transparent CMRstack reside in the electromagnetic bandgap of the reflector and areforbidden to propagate through the stack 517 and onto the metal contact512. The evanescent field set up inside the CMR stack, 514, due to theoptically transparent reflector, allows a very low loss opticalreflection 516 back into the GaN material.

For the examples described herein, the transfer matrix method is used todetermine the normal incidence reflectance of photons in the bluewavelength range. The typical examples illustrated in FIG. 5 a and FIG.5 b, are considered in particular. For the prior art approach shown inFIG. 5 a, the reflectivity of different metal electrodes is compared inTable 2 in terms of the power reflectance in the blue wavelength range.From the selection of metals shown, the highest reflecting metals are Agand Al, whereby approximately 11.3% and 16.7% respectively of loss isintroduced at every reflectance, as indicated by 505 in FIG. 5 a. It istypical for light to undergo several reflections in the LED device priorto extraction and hence the overall loss experienced by the photons istypically many orders larger than the base figure quoted.

TABLE 2 Metal Reflectance (%) Au 18.7 Ag 88.7 Ni 16.7 Cr 30.6 Al 83.3

For comparison with the present invention, a conductive CMR stack, asshown in FIG. 5 b, will be demonstrated utilising doped ZnO ofrefractive index n=1.3 and ITO of refractive index n=2.1. Thecomposition of the multilayer stack is highlighted in Table 3, includingmaterial, refractive index and thickness. The layers are listed in orderof proximity to the underlying semiconductor layer 500. Layer numbers 1to 4 listed in Table 3 lie in the CMR stack 514, while layer number 5 isthe metal electrode layer 512 shown in FIG. 5 b. The total CMR thicknessamounts to approximately 270 nm with a reflectance of 98.5%.

TABLE 3 Layer number Material Refractive index Thickness (nm) 1 DopedZnO 1.3 85 2 ITO 2.1 55 3 Doped ZnO 1.3 85 4 ITO 2.1 85 5 electrode - Ag0.15 + j2.5 >50 nm

The improvement in the efficiency of the alternative electrode designsof the present invention over those of the prior art becomes apparentwhen light trapped in the LED device experiences multiple passes in thevicinity of the electrodes. The resultant effect is illustrated in Table4, which shows a comparison of the relative power remaining in a photonafter experiencing multiple reflections in an LED of the presentinvention and an LED of the prior art, respectively. The percentageimprovement in an LED of the present invention over the prior art deviceis indicated in the final column.

TABLE 4 Power remaining in Power remaining in Number of LED of presentprior art LED reflections invention (Ag metal) Improvement 1 0.985 0.88711.0% 2 0.970 0.787 23.3% 4 0.941 0.619 52.0%

Additionally, as illustrated in FIG. 5 a and FIG. 5 b, in both cases thefinite size of the contact inhibits light emitted under the contact fromescaping. However, as illustrated in FIG. 5 b, in the present inventionthis trapped light is not attenuated by the contact, but is allowed toescape after subsequent multiple total internal reflections, eventuallyescaping out of the LED surface as indicated by 517. This allows for anincrease in the total luminous output of the LED.

In a second aspect of the present invention, as shown in FIG. 6, a lightemitting module is formed by embedding a phosphor, 606, in anencapsulating material residing on top of a light emitting device of thepresent invention. The encapsulation comprises two layers ofoptically-transparent and environmentally-resistant material. The firstencapsulation material, 605, that is disposed proximal to the lightemitting device, is formed of a high refractive index material. Thematerial is preferably textured or shaped to extract the maximum numberof photons, but need not be. The encapsulant 605 can comprise a siliconematerial having a refractive index of approximately 2.1, 2.0, 1.8, or1.6, but may have other refractive index or comprise other material. Ahigh refractive index encapsulant allows a larger amount of light toescape from the roughened surface of the LED due to the larger escapecone angle. Texturing of the encapsulant can also be used to alter thefar-field radiation profile of the LED. This may be in the form of aFresnel lens or microlens texturing.

The phosphor is subsequently disposed on the high refractive indexencapsulant 605, while another encapsulation material 607, distal to thelight emitting device, is disposed on the top of the phosphor. Thephosphor 606 is located proximal, but not in contact with, the top n-GaN604 LED material to avoid thermal heating of the phosphor and improveits lifetime. The phosphors may comprise of, but are not limited to,YAG:Ce, phosphors based on II-VI materials such as selenides, telluridesand sulphides and ZnS and InP, and also Europium doped Silicatephosphors, and Cerium and Terbium doped oxides and nitrides. In theseexamples, the combined emission from the phosphor and the LED aredesigned to emit the desired colour.

The distal encapsulant 607 can also be shaped to provide the desired farfield emission from the light emitting module. The shaping can vary fromhorizontally flat to hemispherical, for Lambertian emission, toparabolic for directional emission, and to other more complex shapes formany different desired far-field emission profiles. The encapsulant 607also acts as a passivation layer to protect and seal the LED fromenvironmental factors. Additionally, the encapsulant 605 and 607 maycomprise the same material. Another passivation layer may also residebetween the first encapsulation layer 605 and the LED device, asdescribed in the method of manufacture below. The encapsulants cancomprise, but are not limited to, silicone gels and resins andelastomers, ABS resins, epoxy, acrylates, spin on glass, PMMA, andthermoplastics and thermosetting resins.

An adhesion layer and reflector layer, 606, such as Ni/Au, or Ag isdeposited on top of layer 609. A metal contact, 610, is placedsubsequently on layer 608. This acts as an electrode to spread currentacross the surface on the LED, as well as a region with an adhesionsurface for wire bonding. Electrical contact is made by wire 611contacting the top electrode, while the circuit is closed by makinganother contact with the conductive carrier 600.

The light emitting module may be used for applications where whiteillumination is required, such as in solid state lighting. Other usesinclude architectural, medical or signage applications.

In a preferred embodiment of the present invention, the bottom and topCMR stack layers allows the light emitting device to be thinned down toa total thickness of less than 5 μm, preferably less than 3.5 μm, morepreferably less than 3.0 μm, still more preferably less than 2.5 μm,even more preferably less than 2.0 μm, and most preferably less than 1.0μm. This allows the LED to achieve high extraction efficiency whilestill maintaining high current injection. For example, if the LEDstructure of FIG. 1 is revisited, layers 104, 103 and 102 of the LEDwaveguide structure are reduced in thickness, thereby allowing fewerconfined modes to reside in the waveguide and hence more efficient lightextraction. Typically, a 3 micron structure will possess approximately44 trapped modes while a thinner 1 micron LED structure can sustainapproximately 15 trapped modes. Fewer modes allow surface extractiontechniques such as roughening to perturb a large percentage of thetrapped mode residing in the waveguide and hence will enable efficientcoupling of the trapped mode into leaky modes and subsequent extractionover a short propagation distance.

One drawback of reducing the thickness of the LED device is thereduction in the ability for the n-doped material 104, such as n-GaN, tospread carriers horizontally, which ultimately causes current crowdingin the layer 104 and thus affects the injection efficiency of the LED.It is an object of the present invention that, by using the low lossconductive CMR stacks, an increased percentage coverage of the topelectrodes is introduced to minimise current crowding and allow improvedcurrent injection and high drive current in thin LED devices. Topelectrodes can be introduced at intervals of a minimum of 100 μm, 80 μm,60 μm, 40 μm, or 20 μm.

Method of Manufacture

In the final aspect of the present invention a method of manufacture ofa vertical LED structure with a metallic substrate and reduced lossmetal contact regions is proposed. The metal substrate provides bothgood thermal and electrical conduction during LED operation. Otherbenefits of vertical LED structures arise from the existence of one topmetal contact rather than two, which effectively increases the activetop emission area. The large area bottom metal contact also improves theelectro-static discharge capabilities of the device. An example of thecomplete process is shown in FIGS. 7 a to 7 g.

FIG. 7 a shows an example of a GaN based LED epitaxially grown on asuitable growth wafer 700. The growth wafer can comprise, but is notlimited to, Sapphire, Silicon Carbide, free-standing GaN or any otherlattice matched material. The growth wafer can also comprise Si, as thisis particularly beneficial when moving to larger six inch (152.4 mm)wafer diameters.

The LED device comprises at least n-type semiconductor layer 701,followed by an active light emitting region 702, subsequently followedby a top p-doped semiconductor material 703. The active region 702 cancomprise a single quantum well (QW) region or multiple quantum wells(MQW). These layers are grown by conventional semiconductor growthtechniques, such as metal organic vapour phase epitaxy (MOCVD),molecular beam epitaxy (MBE), or alternatively atomic layer deposition(ALD).

In the case of an n-GaN layer 701, the layer can have a thickness ofabout 0.5 μm, about 1.5 μm, about 2.0 μm, about 2.5 μm, about 3 μm orabout 4 μm. The MQW region 702 may comprise InGaN/GaN or AlGaN/GaNmultilayer stacks. When these layers are forward biased they can emitlight in the wavelength range between 240 nm and 680 nm. In the case ofthe p-doped GaN layer, the thickness can vary between 5 nm and 400 nm,for example about 50 nm, about 100 nm, about 150 nm, or about 180 nm. Itis important to note that the structure will be inverted and hence thecurrent top surface will reside at the bottom of the device once theprocessing steps are complete.

As shown in FIG. 7 b, the bottom transparent CMR stack is thendeposited. This can be carried out by any conventional depositiontechnique such as, but not limited to, any chemical vapour depositiontechnique (CVD), including low-pressure chemical vapour deposition(LPCVD), plasma-enhanced chemical vapour deposition (PECVD), atomiclayer deposition (ALD), or other techniques such as sputtering orevaporation. A first metallic low resistivity contact layer 705 issubsequently deposited on the structure. This layer also acts as a goodadhesion layer between the metal permanent substrate and the underlyingp-doped GaN or semiconductor region. This contact region preferablycomprises one or more of Ni/Au, Ti/Au, Cr/Au, Au, Pd, Pt, Ru, Ni, Cr,ZnO, CNT, Ag, ITO, Al, and W, although other suitable materials may beused. As a result of the present invention, a larger selection ofconductive adhesion metal layers can be employed, since the reflector704 resides between layer 705 and the p-GaN region 703 and thereforeoptically lossy metallic layers can also be utilised.

The bottom contact region is defined lithographically and transferredinto the transparent CMR stack and the metal layer as shown by 704 and705 in FIG. 7 b. The bottom contact region pattern may be etched by anyetching technique suitable for materials residing in the transparent CMRstack and the metal 705, including wet etching or plasma etching,reactive ion etching (RIE) and inductively coupled plasma (ICP).

Following the definition of the bottom CMR stack, the LED die regionsare defined lithographically and etched to form trenches, 706, isolatingthe individual LED die, as shown in FIG. 7 b. The formation of theindividual LED die prior to flip chip and growth wafer removal allowsimproved stress relief.

As shown in FIG. 7 c, a passivation layer 708 is then allowed to filland overcoat the trenches 706. The passivation layer can comprise, butis not limited to, SiO₂, Si₃N₄, polymer or spin-on-glass. Subsequently,a metal seed layer 707 is grown on top of the adhesion layer 705. Thepassivation layer acts to protect the GaN from environmental factors, aswell as avoid the conductive seed layer shorting the LED structure.Layer 707 provides the seeding for any subsequent chemical platingprocess. This layer can comprise metals such as Cr, Cu, Pt, Au, Ag, Ti,Ni, and Pd.

As shown in FIG. 7 d, an additional metal adhesion layer 709 mayoptionally be deposited on the top surface of the passivation layer 708.The material can be selected from the same metals as for layer 705.Following the optional layer 709, the carrier metal substrate layer 710is overgrown. This layer may comprise, but is not limited to, one of ora mixture of Au, Cu, Ni, Cr, Pt, Pd, In, and Al. The metal is grown byevaporation, sputtering, electroplating or electro-less plating. A thicklayer of metal is formed with a thickness of approximately 20 μm,approximately 50 μm, approximately 100 μm, or approximately 150 μm. Thisthick layer 710 provides a rigid support to hold the underlying LED dieduring removal of the growth substrate 700. The removal process can beachieved by a selective etching process, a lift-off process such aslaser lift-off, or a polishing method. A combination of such devices canalso be implemented.

Following removal of the growth substrate 700, the LED dies are flipped,as shown in FIG. 7 e. The new permanent carrier is now the thick metallayer 710 and the n-doped GaN or semiconductor material is the topexposed surface. The top conductive CMR stack is then deposited. Thedifferent layers in the multilayer stack are successively depositedusing processes similar to those utilised for the bottom transparent CMRreflector. Subsequently, an adhesion layer is also deposited to assistwith the bonding of the final top metal contact. An additionallithography and etch step is required at this stage to define the shapeof the top transparent CMT stack. This will comprise similar processingsteps to those used for CMR stack 704.

If, during the deposition of the transparent CMR reflector, one or moredielectric materials are introduced, then an overcoating, conductive,optically-transparent layer is deposited to allow electrical conductionbetween the top metal contact 714 and the N-doped GaN layer 701, asshown in FIG. 1. The selection criteria for suitable materials anddeposition techniques for the overcoating layer are identical to thoseof the top and bottom CMR layers. Following deposition, anotherlithography mask pattern transfer and an etch step is included toredefine the location of the overcoated top contact region and to removeany conductive layer that extends further than one of 1 μm, 2 μm, 3 μm,5 μm, or 10 μm around the perimeter of the transparent CMR reflector712.

Finally, as shown in FIG. 7 e, an n-type metal electrode contact pad 714is patterned on top of the CMR stack 712 using a lift-off process. Alithography step and negative resist are employed to deposit the metalconformally on the top surface of the CMR stack. A wet or dry plasmaetch are used to transfer the pattern into the resist.

In order to improve capability of the LED device to extract light fromthe top surface, the n-GaN material is roughened. This can beaccomplished by many techniques, such as wet anisotropic etching orphoto-assisted wet etching. In this case pyramids, inverted pyramids orwhisker type roughness following the crystal plane of the GaN are formedby use of suitable chemicals, such as KOH. During wet etching, theconcentration, temperature, UV irradiation and biasing of the samplescan all be controlled to assist in roughening the surface. The pyramiddiameter is preferably between 0.5 μm and 2.5 μm.

Alternatively, high refractive index (preferably larger than n=2.0)optically transparent clusters of size approximately 0.5 μm, 1.0 μm, 1.5μm, and 2.0 μm can also be utilised instead of the wet etching process.Nanoclusters of materials such as Si₃N₄ or GaN crystals can be depositedon the surface of the n-GaN to improve light extraction.

Finally, as shown in FIG. 7 g, the individual LED die are separatedalong the trenches 706. The devices can subsequently be packaged withencapsulant and phosphors to produce complete solid state lightingmodules, of the type shown in FIG. 6.

1. A light emitting device comprising: a first semiconductor layerhaving doping of a first type; a second semiconductor layer havingdoping of a second type; a light emitting region interdisposed betweenthe first and second semiconductor layers; a first electrode layerdisposed proximal to the first semiconductor layer and distal to thesecond semiconductor layer; a second electrode layer disposed proximalto the second semiconductor layer and distal to the first semiconductorlayer; and a first multilayer reflector stack interdisposed between thefirst electrode layer and the first semiconductor layer, the firstmultilayer reflector stack extending at least partially across a surfaceof the first semiconductor layer and comprising at least a first layerdisposed proximal the first electrode layer and a last layer disposeddistal the first electrode layer; and a second multilayer reflectorstack interdisposed between the second electrode layer and the secondsemiconductor layer, the second multilayer reflector stack extendingonly partially across a surface of the second semiconductor layer andcomprising at least a first layer disposed distal the second electrodelayer and a last layer disposed proximal the second electrode layer;wherein at least the first and the last layer of the first multilayerreflector stack are electrically conducting and optically transparentand at least the first layer and the last layer of the second multilayerreflector stack are electrically conducting and optically transparent;wherein at least 60% of light incident on the first multilayer reflectorstack that is generated in the light emitting region is reflected by thefirst stack and at least 60% of the light incident on the secondmultilayer reflector stack that is generated in the light emittingregion is reflected by the second stack; wherein the transverse extentof the second multilayer reflector stack is less than that of the firstmultilayer reflector stack; wherein the second multilayer reflectorstack extends across at least the same part of the surface of the secondsemiconductor layer across which the second electrode layer alsoextends; and wherein light generated in the light emitting region isextracted from the device through those parts of the surface of thesecond semiconductor layer opposite to the first multilayer reflectorstack but across which the second multilayer reflector stack does notextend.
 2. (canceled)
 3. A light emitting device according to claim 1,wherein: the first and/or second multilayer reflector stack comprise atleast 3 layers, preferably at least 4 layers, and most preferably atleast 5 layers; and, the first and/or second multilayer reflector stackscomprise at least two different optical refractive indices.
 4. A lightemitting device according to claim 1, wherein at least the first layerand the last layer of the first and/or second multilayer reflector stackare partially in contact in order to achieve electrical conductionbetween the first and last layer of the respective stack.
 5. A lightemitting device according to claim 1, wherein the second electrode layerextends across a proportion of the area of the light extracting surfaceof the second semiconductor layer that is more than 1%, preferably morethan 3%, more preferably more than 5%, even more preferably more than10%, still more preferably more than 25%, and most preferably more than50%.
 6. A light emitting device according to claim 1, wherein the firstand/or second multilayer reflector stack comprises a dielectric materialselected from a group which includes Magnesium Fluoride, SiliconNitride, Silicon Dioxide, and metal oxides, nitrides and/or oxynitridesderived from a group of metals consisting of Aluminium, Hafnium,Tantalum, Titanium, Chromium, and Zirconium.
 7. A light emitting deviceaccording to claim 1, wherein the electrically conducting and opticallytransparent layers in the first and/or second multilayer reflector stackcomprise a material selected from the group consisting of ZnO, IndiumTin Oxide (ITO), GaN, Carbon NanoTubes (CNT), transparent conductivemetal oxide, transparent conductive nitride, transparent conductiveoxide with spinel crystal structure, ITO nanorods, GaN nanocolumns, AlNnanocolumns, ZnO doped with Silicon, transparent conductive polymer,poly(ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS),polyaniline poly(styrenesulonate) (PANI:PSS).
 8. A light emitting deviceaccording to claim 1, wherein the surface of the second semiconductorlayer through which light is extracted is at least partially roughenedto increase the light extraction.
 9. A light emitting device accordingto claim 1, wherein the total combined thickness of the firstsemiconductor layer, the second semiconductor layer and the lightemitting region is less than 3.5 microns, preferably less than 3.0microns, more preferably less than 2.5 microns, even more preferablyless than 2.0 microns, and most preferably less than 1.5 microns.
 10. Alight emitting device according to claim 1, wherein exposed surfaces ofthe first semiconductor layer, the second semiconductor layer and thelight emitting region are at least partially overcoated with apassivation layer to protect the light emitting device.
 11. A lightemitting module for solid state lighting applications comprising: alight emitting device according to any claim 1; a first encapsulatingmaterial covering at least the light extracting surface of the secondsemiconductor layer; a second encapsulating material overcoating atleast the first encapsulating material; and, a phosphor materialinterdisposed between the first and the second encapsulating materials.12. A light emitting module according to claim 11, wherein at least asurface portion of the first encapsulating material distal the lightextracting surface of the second semiconductor layer is textured formaximum light extraction.
 13. A method of manufacturing the lightemitting device of claim 1, the method comprising the steps of: growingeach of the second semiconductor material, the light emitting region,and the first semiconductor material; depositing a first multilayerreflector stack; forming mesa isolation trenches in the light emittingdevice; depositing a passivation layer; depositing a first electrode;attaching a conductive sub-mount; removing growth substrate; depositinga second multilayer reflector stack; depositing a second electrode;roughening a top surface of the second semiconductor layer; andseparating an isolated light emitting die.